1. Field of the Invention
The present invention relates to a method and an apparatus for exposing a photosensitive medium to laser beams having source image information representative of images to be reproduced. More particularly, the present invention is concerned with a method of controlling the intensity of the laser beams for obtaining an ideal density of reproduced images which is similar to a nominal density represented by the source image information, and a laser printer which is suitably adapted to practice the method.
2. Discussion of the Prior Art
An apparatus for imagewise exposing a photosensitive medium to a laser radiation is used in a laser printer, for example, for reproducing original images. Local spots on the photosensitive medium are irradiated by laser beams, which are controlled according to optical image signals supplied from a suitable device such as an image reader, such that the intensity of the laser beams is varied continuously or in steps so that the density of reproduced images coincides with a nominal or ideal density represented by the image signals, i.e., the density of the original images. To expose the photosensitive medium to the thus controlled laser beams, the beams are deflected for scanning the photosensitive medium in an X-axis direction over a predetermined angular range corresponding to a recording width of a recording medium. Thus, the photosensitive medium is exposed at its local spots along a line in the X-axis direction. Each time a scanning of the photosensitive medium by the laser beams in the X-axis direction is completed, the photosensitive medium is displaced in a Y-axis direction perpendicular to the X-axis direction, by a suitable distance to expose a next line parallel to the X-axis. Namely, the X-axis scanning lines are spaced apart from each other by a predetermined scanning interval in the Y-axis direction. Thus, the photosensitive medium is scanned in the X-axis and Y-axis directions by the laser beams controlled by the image signals, so that the original images represented by the image signals are reproduced on a suitable recording medium.
Generally, the intensity of a laser beam has a Gaussian or normal distribution, and the nominal intensity is a peak value of the Gaussian distribution. If the Y-axis laser scanning interval is excessively large, the intensity of the light exposing an area between adjacent two X-axis scanning lines is so low that the intensity of the corresponding reproduced images is higher than the nominal value. If the Y-axis laser scanning interval is excessively small, on the other hand, the intensity of the light exposing the area between the adjacent two X-axis scanning lines is so high that the intensity of the reproduced images is lower than the nominal value. To avoid this inconsistency between the densities of the reproduced images and the original images, the Y-axis scanning interval is determined to be almost equal to a half-width of the Gaussian distribution of the laser beam, so that the intensity of the light between the adjacent two X-axis scanning lines is almost equal to that on the X-axis scanning lines, provided that the intensity of the laser beams on one of the adjacent two X-axis scanning lines is almost equal to that on the other of the two X-axis scanning lines, namely, provided that the density of the original images is substantially constant in the Y-axis direction.
The graphs in FIG. 10 show a distribution of light intensity Exp in the Y-axis direction and a distribution of density Dp of a reproduced image, where the adjacent two laser beams have the same intensity at respective positions Y1 and Y2 in the Y-axis direction, which positions are spaced apart from each other by the Y-axis scanning interval "d", which is equal to the half-width of the Gaussian distribution. It will be understood from the graphs that the light intensity and the reproduced image density at an intermediate position between the scanning positions Y1 and Y2 are substantially equal to those at the scanning positions Y1 and Y2.
However, when the density of the original images varies in the Y-axis direction, the density or tone gradation of the reproduced images tends to be different from that of the original images represented by the image signals.
Described more particularly by reference to FIG. 11 which shows a cyclic variation in the density Do of original images in the Y-axis direction, an image reader which is adapted to read the original images produces image signals, and a photosensitive medium is imagewise exposed to laser beams controlled by the image signals, according to a known method of light exposure. The density Dp of the reproduced images is indicated in solid line in FIG. 12, while the nominal density Do of the original images is indicated in broken line in the same figure. As indicated in FIG. 12, the highest and lowest density levels of the images reproduced according to the known exposure method are lower and higher than those of the nominal density represented by the image signals. That is, the amount of variation in the density Dp is smaller than the amount of variation in the nominal density Do. This phenomenon occurs for the following reason.
Where the density Do of the original images varies in steps in the Y-axis direction as indicated in FIG. 6 such that the density at a position Y.sub.n is lower than those at adjacent positions Y.sub.n-1 and Y.sub.n+1, the intensity Exp of the exposure light and the density Dp of the reproduced images are as shown in FIG. 13. Namely, the average density Dp in a local spot En exposed by the intermediate laser beam at the central position Y.sub.n is higher than the nominal or ideal value indicated in broken line. On the other hand, where the density Do of the original images varies in steps as indicated in FIG. 8 such that the density at the central position Y.sub.n is higher than those at the adjacent positions Y.sub.n-1 and Y.sub.n+1, the intensity Exp of the exposure light and the density Dp of the reproduced images are as shown in FIG. 14. Namely, the average density Dp in the local spot En is lower than the nominal or ideal value indicated in broken line. It is noted that in FIGS. 13 and 14, the density Dp increases in the downward direction.
The nominal or ideal value Do of density of the original images indicated above corresponds to the density represented by the image signals, which is indicated in broken line in the graphs of the reproduced image density Dp in FIGS. 13 and 14. In the case of FIG. 8, the density Do at positions that are adjacent to the positions Y.sub.n-1 and Y.sub.n+1 and remote from the central position Y.sub.n is higher than that at the central position Y.sub.n. In this case, the average densities Dp of the reproduced images in the local spots E.sub.n-1 and E.sub.n+1 are higher than the nominal values, whereby the densities Dp of the reproduced images at the positions Y.sub.n-1, Y.sub.n and Y.sub.n+1 are made closer to each other, as indicated in FIG. 9.